Supercritical Power Cycle Research Articles

Thermodynamic and exergy assessment of a biomass oxy-fuel combustion with supercritical carbon dioxide cycle under various recycle flue gas conditions

The ongoing development of second-generation oxy-fuel combustion system with higher oxygen concentration have presented a potential pathway for efficient biomass utilization. It also signified the meticulous role of recycled flue gas in altering the combustion behaviour in the combustor. Accordingly, investigation into different recirculation modes focusing on recycle ratio, wet/dry conditions, and excess oxygen ratio are conducted with an indirect supercritical power cycle integrated biomass oxy-fuel combustion system. Energy and exergy analyses, as well as composition of flue gas produced are presented. Energy and exergy efficiencies increased with reducing recycle ratio, with respective highest efficiency of 31.33% and 44.37% obtained at excess oxygen ratio of 1.01 and recycle ratio of 0.5 under wet condition. Investigation of CO2 and H2O concentration in produced flue gas depicted a separated stage profile under dry recycle condition, different from the slight diverging profile observed under wet condition when the recycle ratio varied. Concentration of NOx and SOx increased with decreasing recycle ratio and are generally higher under dry condition. Selective exergy analyses of the proposed power cycle presented highest exergy loss in the combustor, followed by the air separation unit, heat exchanger, CO2 compression and purification unit, and turbine generator. Based on the obtained result, wet recycle flue gas enhanced the combustion performance in the combustor but slightly impaired the convective heat transfer in the heat exchanger. The performance of proposed biomass power cycle is optimized at low recycle ratio of 0.5 under wet recycle condition.

Advances in solar thermal power plants based on pressurised central receivers and supercritical power cycles

This work addresses the comparative thermo-economic study of different configurations of solar thermal power plants, based on supercritical power cycles and pressurised central receiver systems. For all the cases examined, two innovations are introduced in the solar subsystem, compared to other similar studies. Firstly, the heat transfer fluid in the receiver is either a pressurised gas or a supercritical fluid. Secondly, the receiver is composed of compact structures performing as absorber panels, arranged in a radial configuration. The investigation considers different supercritical CO2 recompression cycles of 50 MWe, including a novel proposal of a directly coupled cycle with heat input downstream of the turbine. Furthermore, the study evaluates different heat transfer fluids in the receiver, specifically CO2, N2, and He, concluding that the former is preferred due to its better thermal performance.The main results show that an increase in the receiver inlet pressure yields to a reduction in its size, favouring the thermal efficiency but penalising the optical efficiency of the solar field. Therefore, optimal working pressures may exist for each configuration, depending on the operating temperature. When comparing the optimal configurations, it is observed that the plant based on the intercooling cycle demonstrates the highest overall efficiency, reaching 32.05%. At last, an economic analysis is conducted to assess the viability of the identified optimal configurations. In this regard, the plant based on the partial-cooling cycle exhibits the lowest levelised cost of electricity at 0.15 $/kWh. This is primarily due to its lower investment cost. The innovative directly coupled cycle follows closely with a cost of 0.17 $/kWh, driven by its high electricity production resulting from its low self-consumption.

Thermoeconomic Analysis of Concentrated Solar Power Plants Based on Supercritical Power Cycles

Solar thermal power plants are an alternative for the future energy context, allowing for a progressive decarbonisation of electricity production. One way to improve the performance of such plants is the use of supercritical CO2 power cycles. This article focuses on a solar thermal plant with a central solar receiver coupled to a partial cooling cycle, and it conducts a comparative study from both a thermal and economic perspective with the aim of optimising the configuration of the receiver. The design of the solar receiver is based on a radial configuration, with absorber panels converging on the tower axis; the absorber panels are compact structures through which a pressurised gas circulates. The different configurations analysed keep a constant thermal power provided by the receiver while varying the number of panels and their dimensions. The results demonstrate the existence of an optimal configuration that maximises the exergy efficiency of the solar subsystem, taking into account both the receiver exergy efficiency and the heliostat field optical efficiency. The evolution of electricity generation cost follows a similar trend to that of the exergy efficiency, exhibiting minimum values when this efficiency is at its maximum.

Preliminary design and optimization of a solar-driven combined cooling and power system for a data center

Aiming at the demand of reducing cost, increasing efficiency and green low-carbon development in data center, a novel solar-driven combined cooling and power system is constructed by coupling supercritical power cycle with transcritical refrigeration cycle. The heliostat field is designed, and the models on thermal, economic, environmental and comprehensive performances are established. On this basis, system performances under design conditions are studied, and the system parameters are analyzed. Thereafter, optimal system performances at typical days in four seasons are compared based on genetic algorithm. The results indicate that: under design conditions, the system energy and exergy efficiencies are 29.47% and 16.21% respectively. The total system capital cost is $1.2576 × 108. The carbon emissions are reduced by 257,420 kg per day. The comprehensive performance evaluation index (CPEI) can reach up to 47.63%. CPEI increases with the increase of turbine inlet pressure. With the increase of high-pressure compressor (HPC) inlet pressure and evaporation temperature, CPEI increases first and then decreases. Furthermore, CPEI shows a continuous downward trend as the load rate of data center increases. The system performances at autumn equinox and spring equinox are more similar, while those at summer solstice and winter solstice are quite different from those of spring equinox.

Determination of the position and characteristics of supercritical heat transfer deterioration

Supercritical fluids (SFs) are widely used in power generation systems using nuclear, solar, geothermal, and fossil energies. The heat transfer deterioration (HTD) phenomenon during heat transfer of SFs is of great importance to the design and operation of the components in supercritical power cycles. The criteria for determining the onset of HTD has been reported in the literature, but the quantitative model to obtain the position of the wall temperature peak during HTD occurrence, and the correlation to accurately predict the heat transfer characteristics at the wall temperature peak, are both missing. In this work, to determine the position and to predict the characteristics of HTD at the wall temperature peak, we developed two correlations based on experimental results for supercritical CO2 (sCO2) in vertical upward tubes with wide range of experimental parameters. The first correlation to predict the location of the peak is developed using experimental data for sCO2, which can accurately determine the dimensionless bulk fluid temperature corresponding to the wall temperature peak. Moreover, a new heat transfer correlation is developed for the Nusselt number at the wall temperature peak, which has a simple form of NuHTD=0.00796Reb0.7215Prb−0.1568K−0.115, where Reb, Prb, and K are Reynolds number, Prandtl number, and supercritical K number, respectively. This correlation shows smaller prediction error compared with five existing correlations in the literature for not only sCO2, but also various other working fluids. It also outperforms the recently proposed K number correlation in predicting characteristics at the wall temperature peak during HTD conditions. The two correlations developed in this work provides the quantitative models for the HTD phenomenon, and can benefit the engineering application of supercritical power cycles.

Experimental and modelling study of syngas combustion in CO2 bath gas

Syngas produced from coal and biomass gasification has been proposed as a potential fuel for direct-fired supercritical power cycles. For instance, the Allam-Fetvedt cycle can offer price-competitive electricity production with 100 % inherent carbon capture while utilizing CO2 dilution of about 96 %. In this work, ignition delay times (IDTs) of syngas have been measured in CO2 diluted conditions using a high-pressure shock tube at two pressures (20 and 40 bar) over a temperature range of 1100 – 1300 K. Syngas mixtures in this study were varied in equivalence ratio and H2:CO ratios. The datasets were compared against the predictions of AramcoMech 2.0 and the University of Sheffield supercritical CO2 2.0 (UoS sCO2 2.0) kinetic models. Quantitative comparative analysis showed that the UoS sCO2 2.0 was superior in its ability to predict the experimental IDTs of syngas combustion. We found that the reaction of CO2 and H to form CO and OH caused the separation of H2 and CO ignition in two events, which increased the complexity of determining the IDTs. We investigated this phenomenon and proposed a method to determine simulated IDTs for an effective comparison against the experimental IDTs. The chemical kinetics of syngas combustion in a CO2 and N2 bath gas are contrasted by sensitivity and rate-of-production analyses. By altering the ratio of H2 and CO as well as mixture equivalence ratio, this work provides vital IDT data in CO2 bath gas for further development and validation of relevant kinetics mechanisms.

Thermal efficiency gains enabled by using CO2 mixtures in supercritical power cycles

The present paper explores the utilisation of dopants to increase the critical temperature of Carbon Dioxide (sCO2) as a solution towards maintaining the high thermal efficiencies of sCO2 cycles even when ambient temperatures compromise their feasibility. To this end, the impact of adopting CO2-based mixtures on the performance of power blocks representative of Concentrated Solar Power plants is explored, considering two possible dopants: hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4). The analysis is applied to a well-known cycle -Recuperated Rankine- and a less common layout -Precompression-. The latter is found capable of fully exploiting the interesting features of these non-conventional working fluids, enabling thermal efficiencies up to 2.3% higher than the simple recuperative configuration. Different scenarios for maximum cycle pressure (250–300 bar), turbine inlet temperature (550–700 °C) and working fluid composition (10–25% molar fraction of dopant) are considered. The results in this work show that CO2-blends with 15–25%(v) of the cited dopants enable efficiencies well in excess of 50% for minimum cycle temperatures as high as 50 °C. To verify this potential gain, the most representative pure sCO2 cycles have been optimised at two minimum cycle temperatures (32 °C and 50°C), proving the superiority of the proposed blended technology in high ambient temperature applications.

Geothermal power plants with improved environmental performance: assessment of the potential for an Italian site

Geothermal energy is a clean resource, which could significantly contribute to the reduction of greenhouse and other gas emissions by replacing fossil fuels for power generation. In many geothermal sites, the resource contains substantial Non-Condensable Gases (NCGs: CO2 and contaminants), whose emissions can be limited to developing power plant schemes suitable for complete resource reinjection. Organic Rankine or other closed-loop cycles are definitely favored in this light. This work investigates a solution for complete NCG reinjection in the liquid-dominated reservoir conditions typical of the Monte Amiata area (Italy), referring to the specific site of Torre Alfina (IT) which presents a specific attractiveness because of its high pressurization. The solution considered avoids flashing the resource and thus presents an appealing environmental performance. The power plant models include energy and exergy balances, as well as exergo-environmental analysis. The overall environmental performance is evaluated by a simplified (preliminary) Life Cycle Analysis (LCA). Different solutions are compared, considering the possibility of sub- or super-critical power cycles.

Exergy-based evaluation of a waste heat driven polygeneration system with CO<SUB align="right">2 as the working fluid

A waste heat driven polygeneration system that couples a supercritical power cycle with a transcritical refrigeration cycle is proposed and evaluated in this study. The stand-alone system using carbon dioxide as the working fluid produces power, refrigeration, and heating capacities simultaneously. Three scenarios of the system with different ambient temperatures (25-35°C) are optimised using exergoeconomic approach to obtain the lowest average cost of the products. The obtained results show that the average product cost is the highest ($113/GJ) for the scenario with the ambient temperature of 30°C, while the product cost is reduced to $82/GJ when the ambient temperature is 35°C. Moreover, the pressure for merging the sub-systems influences the performance of the system significantly, and the variations of the operating conditions differ among the scenarios by increasing the merging pressure. Besides, a multi-objective optimisation is implemented for supporting decision-makers to find the optimal solutions.

Exergy-based evaluation of a waste heat driven polygeneration system with CO<SUB align="right">2 as the working fluid

A waste heat driven polygeneration system that couples a supercritical power cycle with a transcritical refrigeration cycle is proposed and evaluated in this study. The stand-alone system using carbon dioxide as the working fluid produces power, refrigeration, and heating capacities simultaneously. Three scenarios of the system with different ambient temperatures (25-35°C) are optimised using exergoeconomic approach to obtain the lowest average cost of the products. The obtained results show that the average product cost is the highest ($113/GJ) for the scenario with the ambient temperature of 30°C, while the product cost is reduced to $82/GJ when the ambient temperature is 35°C. Moreover, the pressure for merging the sub-systems influences the performance of the system significantly, and the variations of the operating conditions differ among the scenarios by increasing the merging pressure. Besides, a multi-objective optimisation is implemented for supporting decision-makers to find the optimal solutions.

Proposal of a new design of source heat exchanger for the technical feasibility of solar thermal plants coupled to supercritical power cycles

Solar thermal power plants coupled to supercritical CO2 cycles seems to be a way to increase the global solar-to-electric efficiency. For that, the concentrating solar technology that is best integrated is the molten salt central receiver with a thermal energy storage associated. This work is focused on one of the main challenges of this scheme: the source heat exchanger transferring the thermal energy from the molten salt in the solar field to the CO2 in the power cycle. A new design, based on the printed circuit heat exchanger technology is proposed, that withstands the pressure difference and avoids the molten salt plugging when circulating through microchannels. The thermo-mechanic model of this heat exchanger is also calculated.This work also addresses a thermo-economic optimization of the printed circuit heat exchanger proposed. For that, it is considered the global performance of the solar thermal plant for three layouts: recompression, intercooling and partial-cooling cycles. This optimization yields to a great reduction in the investment cost of these source heat exchangers, achieving the lowest cost in the partial-cooling configuration, followed by the intercooling and finally, the recompression. This trend is also observed in the global performance of the solar plant, so the partial-cooling layout is the one with the lowest levelized cost of electricity; this value is similar to that of the intercooling layout, and both are well below from the cost in the recompression layout, which results the most expensive configuration.

Exergoeconomic investigation of a multi-generation system with CO2 as the working fluid using waste heat

A combined cycle coupling a supercritical power cycle with a transcritical refrigeration cycle with carbon dioxide as working fluid is evaluated and optimized from the exergoeconomic viewpoint. The system is designed to produce power, refrigeration, and heating simultaneously. If no net power is generated, the system can be described as a cogeneration. First, the cogeneration system working with various evaporation temperatures is optimized for having the lowest average cost of the products, which indicates that the system is preferred to operate at the lower evaporation temperature. Then for tri-generation, by increasing the power output and the pressure merging two sub-systems, the cost of the products (average and individual) reduces and the efficiency of the overall system increases. In addition, the cost of the power is the lowest while the cost of the refrigeration is the highest, and the cost of the heating is sensitive to the operation conditions of the overall system.

A numerical prediction on heat transfer characteristics from a circular tube in supercritical fluid crossflow

Studies on fluids at supercritical pressure have been developed in a rapid-growth manner so that geometries are not restricted to circular tube. This paper is devoted to conduct a brief review of the recent main experiments on supercritical fluids such as CO2, water, R134a and n-decane, and to comment the findings. In addition, this paper discusses the probability of supercritical fluids flowing across tube bundles for the frequent use in shell-and-tube heat exchangers in supercritical power cycle systems. A numerical prediction of flow patterns and heat transfer characteristics of a circular tube in supercritical water crossflow is carried out to make a fundamental investigation of the application of supercritical fluids on external flows. Results indicate that the influence of heat flux on heat transfer in crossflow is similar with that for in-tube flows. The thermal physical properties significantly affect the heat transfer and the flow patterns also show unique characteristics with the change of boundary conditions.

Comprehensive evaluation of supercritical power cycle system using non-azeotropic mixtures driven by medium-temperature solar heat source

Aimed at the medium-temperature solar energy heat source, the use of low-boiling binary non-azeotropic organic mixtures as the supercritical cycle working fluids to generate power was put forward. On the premises of meeting the requirements of thermal property, safety, and economic efficiency, 8 working fluids combining R245fa with R152a according to different mass ratios were used and the supercritical thermal system mathematical model was established based on the first and second laws of thermodynamics. The cycle characteristics were contrasted from the multi-angle aspects including seven indices like thermal efficiency, exergy efficiency, and so on. Considering various factors, the multi-index comprehensive evaluation model of energy efficiency was constructed by using the analytic hierarchy process method. Through simulation calculations, the results show that properly increasing the proportion of R245fa promotes effective improvement of the supercritical cycle characteristics and working fluid R245fa/R152a(0.96/0.04) can improve the indexes including net output power, economy, and environmental performance of the system. The proposed one has the optimal evaluation index value among the candidates.

A supercritical power cycle using carbon dioxide for low temperature geothermal sources

A supercritical power cycle using carbon dioxide for low temperature geothermal sources

Design and optimisation of linear Fresnel power plants based on the direct molten salt concept

Abstract Thermal energy storage (TES) is an exclusive feature of solar thermal electric (STE) plants, allowing for solar energy to be efficiently stored and dispatched to the grid at a chosen point in time, thus providing an enhanced operation flexibility while potentially reducing the levelised cost of electricity generation (LCoE). The direct molten salt (DMS) concept, where a liquid nitrate/nitrite salt mixture is used both as heat transfer fluid in the solar field and as sensible heat storage medium in the TES represents currently the most promising technological option, in terms of LCoE optimisation and technology maturity, for STE plant with thermal energy storage. Here, different DMS plant configurations based on the linear Fresnel solar collector technology are investigated targeting to define optimal designs with regards to the LCoE. For this purpose, a simulation tool has been developed capable of describing the dynamic plant behaviour by means of best estimate methods to determine the annual yield and operational availability of linear focusing DMS STE plant, together with estimating the investment and annual operation costs of such plants. Simulations results from this tool have been validated against a well established thermo-hydraulic simulation tool (TRACE). The study concentrated on the following aspects: 1. Identification of optimal operational parameters of a STE plant with TES coupled to a power cycle 2. Economical evaluation of the chosen plant concepts and definition of optimal plant configurations 3. Investigation of LCoE reduction perspectives by means of higher operation temperatures These analyses exhibit for the given plant setup an optimum choosing a sub-critical reheat power cycle at 540 °C/165 bar and the so-called “Hitec” molten salt mixture as heat transfer fluid (HTF). However supercritical power cycles in a DMS STE plants are not considered and may bring further LCoE reductions. Additionally, in terms of LCoE, the study reveals an optimal plant capacity in the range of 100 MW el to 150 MW el . Finally, the study shows that a further increase in the temperature of both power cycle and solar field beyond the previously named optimal values leads to increased LCoE.

Recuperated power cycle analysis model: Investigation and optimisation of low-to-moderate resource temperature Organic Rankine Cycles

A numerical model for recuperated power cycles for renewable power applications is described in the present paper. The original code was written in Python and results for a wide range of working fluids and operating point conditions are presented. Here, the model is applied to subcritical and transcritical Rankine cycles. It comprises a brute-force search algorithm that covers a wide parametric study combining working fluid, resource and cooling temperatures as well as high-side pressures in order to ascertain the best working fluid for a given resource temperature and operating point. The present study determined the fluids that maximise the specific energy production from a hot stream for a range of low-to-medium temperature (100–250 °C) resources. This study shows that for the following resource temperatures: 100 °C, 120 °C, 150 °C, 180 °C and 210 °C, R125, R143a, RC318, R236ea and R152a were found to maximise specific energy production, respectively. In general, the inclusion of a recuperator within the power cycle results in greater specific energy production for a given operating temperature. However, it was found that for all fluids there was a threshold pressure above which the inclusion of a recuperator did not enhance system performance. This may have design and economic ramifications when designing next-generation transcritical and supercritical power cycles.

Cross industry integration for power plants that utilize hydrocarbon fuels and supercritical fluids

This manuscript provides a basis for optimization of supercritical fluid power cycles, a forum for future power plants and a pathway on how currently used supercritical power cycles can have a springboard effect for wider penetration of Supercritical Fluids into energy and other domains. Relaxation of cross industry barriers to entry, modularized platforms, and effective integration are expected to accelerate realization of wider usage of supercritical fluid technologies. Specific case studies include supercritical water power cycle integrated biomedical waste to electricity facility, integrated biomass or coal conversion to liquid fuels and power plants, and urea/ammonia production in integrated gasification combined cycle power plants.

Study on Heat Transfer in Heat Exchangers for a New Supercritical Organic Rankine Cycle

Electrical power from geothermal heat is hardly efficient, if the temperature level of geothermal water is low. This situation is predominant in middle Europe, for example, in Germany, where even in hot-spot areas like Norddeutsches Becken, Oberrheingraben, and Molassebecken the temperature of geothermal water is less than 160°C. For this, efforts have to be made in optimizing the power cycle efficiency. Supercritical fluids provide a higher net energy output per unit mass than subcritical fluids but their physical properties strongly depend on temperature variations, especially close to the supercritical point. Based on the requirements of a new modular and mobile supercritical power cycle MONICA (modular low temperature power cycle Karlsruhe), which will be built within the next few years, different heat exchanger types are investigated within this study in order to determine the most compact design with respect to mobility, mountability, and efficiency. Changes in physical properties of propane, the working fluid of the new cycle, are taken into account by iterative, stepwise calculation of heat exchanger types like double-pipe, shell-and-tube, and plate heat exchangers. For this, common Nusselt number correlations are implemented in the stepwise iteration. Influence of geometry on flow conditions and analysis of part load sensitivity are provided.

Parametric study and optimization of an ejector-expansion TRCC cycle integrated with a water purification system

A combined cogeneration cycle is proposed in which the waste heat from an ejector-expansion trans-critical CO2 refrigeration cycle is utilized for power production and water purification simultaneously. The waste heat utilization was performed by means of a CO2 supercritical power cycle and a desalination system for pure water production. In order to run the desalination system, an absorption heat transformer is employed to upgrade the lower temperature waste heat. Thermodynamic models are developed for both the ejector-expansion trans-critical CO2 cycle and the combined cogeneration cycle consisting of all the above mentioned cycles. A parametric study is conducted to investigate and optimize the performance of each cycle under various operating conditions. It is found that, at the optimum conditions, the energy efficiency ratio of combined cogeneration cycle is about 13–45% higher than the coefficient of performance of the ejector-expansion trans-critical CO2 cycle. It is also concluded that, as the gas cooler temperature increases, the pure water production rate in the combined cogeneration cycle is increased.